1. Field of the Invention
The field of invention relates to high speed scanning of an optical delay and more specifically to a method and apparatus for performing high-speed scanning of an optical delay for an Optical Coherence Tomography (OCT) imaging system.
2. Background of the Related Art
Optical coherence tomography (OCT) is a new imaging modality. OCT has the ability to perform high resolution, high-sensitivity, cross sectional imaging of microstructures. The use of OCT has several significant advantages over standard optical imaging techniques and ultrasound. First, OCT can directly measure cross-sectional microstructure on a micron scale. Second, OCT can perform imaging of structures in situ and without contact. Third, imaging can be performed in real time, and fourth, OCT technology is fiber optically based and can be interfaced with a wide range of medical, microscopic, or industrial applications.
OCT is analogous to ultrasound B mode imaging, except that it uses light rather than sound and performs imaging by measuring the backscattered intensity of light from a microstructure. OCT produces one, two, or three dimensional images by directing an optical beam at an object to be imaged, and measuring backscattered light as the beam is scanned across the object. The OCT image is a gray scale or false color two dimensional representation of backscattered light intensity in a cross sectional plane. In medical imaging, the OCT image represents the differential backscattering contrast between different tissue types on a micron scale.
There are a variety of interferometric embodiments for OCT systems. One typical implementation uses a fiber optic coupler for the Michelson interferometer. One of the arms of the interferometer is used to deliver and scan the optical beam on a sample, while the other arm functions as a reference arm and has a high speed longitudinal scanning mechanism. When the path-length to a reflection site within the sample is matched to the reference arm path-length, optical interference occurs at the photodetector. The interference signal is detected, demodulated, processed, and stored and/or displayed to yield the backscattered light intensity versus depth for a given transverse position of the incident beam.
The longitudinal scanning mechanism is a key technology for OCT systems. A requirement of many OCT systems is to achieve a near uniform-velocity longitudinal scanner, with high-speed, and high duty cycle. Presently, most commercial OCT systems use either a small moving retro-reflector mounted onto a galvanometric beam steerer or a fiber stretcher using PZT actuators. Techniques demonstrated to date do not meet system needs for certain applications that require high-speed scanning, such as real-time medical endoscopic procedures.
Previous work has identified the concept of a rotating CAM to perform longitudinal scanning. Hecht (U.S. Pat. No. 3,776,637) has described the use of a circular involute reflector for providing a variable path-length reference arm, and its use in an interferometer for a Fourier spectrometer. Hecht, however, fails to describe the use of this concept in an OCT system and the unique features associated with this use, nor does Hecht disclose any other rotating mechanisms for delay line scanning. Tasaka (Japanese Patent Application No. 58-24005) discloses a CAM for use as a potentiometer and its use with a measuring interferometer. Tasaka, however, describes a radius that varies in proportion to the angle of rotation. Such a device will not ensure a retro-reflected beam, which is a condition necessary for OCT. Marantette (U.S. Pat. No. 5,387,969) relates to using a rotating CAM for blocking light incident on a photo-detector and other applications. Morris et al. (U.S. Pat. No. 5,007,721) is directed to a mechanically rotated Doppler frequency shifter that uses a spiral embodiment for path-length adjustment. The embodiment described therein is very difficult to fabricate. Frangineas (U.S. Pat. No. 5,033,853) discloses an apparatus for autocorrelating optical radiation signals which is used in an optical correlator. The basic embodiment of this device, however, cannot be driven to high speeds because of the required linkage to a moving corner cube.
Presently, there are no commercially available high-speed longitudinal (path-length) scanning devices for use in OCT systems, despite the intense need for such devices to address important medical markets. None of the related art addresses details on the use of a rotating element, such as a CAM (or other devices described in this disclosure), in an OCT system, nor do they address the unique optical alignment issues, manufacturing issues, and timing, calibration, and correction techniques required for use in OCT systems. Further, the related art fails to address critical design issues such as the need for techniques to fabricate, calibrate, and overcome operational imperfections in real-time to achieve high resolution OCT imaging capability, or other aspects unique to OCT systems.
As described above, the related art has various disadvantages. First, the disclosed systems are not scalable to the faster image rates required for medical and other "high-end" markets where motion induced artifacts from a slow imaging rate can destroy resolution or throughput speeds in on-line nondestructive evaluation (NDE) applications. Also, an inherent difficulty in most of the related art is the repetitive starting and stopping a mass at high speed. This can cause non-uniform scanning velocity which requires non-uniform Doppler signal processing requirements, forcing complex receiver circuitry and, more importantly, significant inefficiencies in signal-to-noise ratio and image acquisition rate. Degradations can exceed 10 dB from ideal. Also, retro-reflectors can be massive and therefore high speed cannot be achieved. Additionally, the translation from the angular motion of a Galvanometer shaft to displacement of the retro-reflector is not linear.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.